Perhaps the best thing about the metric system (more formally, the International System of Units) is the way in which it allows for the easy estimation of many practical problems, and takes advantage of the intuitive connections people can make.
For instance, one millilitre (mL) of volume occupies a space of 1 centimetre (cm) by 1 cm by 1 cm. It is easy to imagine a cube that is 1 cm to a side, so it is easy to imagine what 1 mL of liquid would look like. Similarly, knowing that 1 litre (L) of water (a very familiar quantity of matter) has a mass of 1 kilogram (kg) allows a person to pretty easily consider what the weight of something in kilograms might be. A volume of 1000 L of water weighs one metric tonne, and occupies a space of one cubic metre.
The metric connections extend to other elements of science and everyday life. The metric temperature scale is well suited to a planet where water is exceptionally important. While calories are not strictly a metric unit, they do tie usefully into the common theme of water, with one calorie being the amount of energy required to heat 1 cubic centimetre (equivalent to 1 gram, and to 1 mL) of water by 1ËšC. Note that ‘calories’ as expressed in relation to food are usually kilocalories: the amount of energy required to heat 1 L of water by 1ËšC.
All very useful!